ALD is a method in which deposition of an atomic layer of material is controlled by a pre-deposited layer of a precursor. ALD processes typically promote the adsorption of gases into a substrate and hence the deposition of an atomic layer of material on the substrate. By alternating the supply of a reaction gas and a purging gas, ALD processes can uniquely control the deposition of material on a substrate at an atomic level. For example, when a reaction gas (or precursor) is exposed to a substrate surface, atoms of the reaction gas are chemically adsorbed into the substrate. The reaction gas is then purged by exposing the substrate to a purging gas. The purging gas ideally only reacts with the substrate where the reaction gas had been previously adsorbed. The resulting chemical reaction eventually forms an atomic layer of material onto the substrate surface. ALD thus provides unique control of the doping dosage and doping location at an atomic level, allowing for selective layer growth and both single and double layer dopant coverage.
Conventional ALD applications aid in meeting micro-scaled production requirements. ALD applications are typically ideal for abrupt, localized highly doped structures with relatively sharp profiles and provide little or no interaction with the growing layer. Conventional ALD doping, in a hydrogen gas ambient, generally provides high segregation of the dopant (in most cases phosphorus) and a broad profile. Thus, conventional ALD processes limit (direct current) DC and (radio frequency) RF performance. Important band engineering factors such as the gain, Early voltage, voltage between the base and collector and cutoff frequency are adversely affected.
Conventional silicon-based bipolar junction transistors (BJTs) have been a dominant semiconductor device since the advent of the integrated circuit. Many other semiconductor materials outperform silicon-based devices. However, because most semiconductors are incompatible with the silicon-based process technologies, the development of such materials has not been forthcoming. Silicon Germanium (SiGe) and Silicon Germanium: Carbon (SiGe:C) have been recent exceptions.
SiGe has an energy gap that varies as a function of the concentration of Germanium. Thus, SiGe allows for band-gap engineering, which in turn provides improvements in high speed and high frequency performance. A principal application of SiGe has been with heterojunction bipolar transistors (HBTs). The base of an HBT is the most heavily doped region of the transistor and is thus a prime area for band-gap engineering. SiGe HBTs generally offer a higher unity gain frequency, lower noise, higher collector currents and better linearity than the conventional silicon BJT. Moreover, SiGe HBTs may be integrated with existing CMOS technologies, keeping production costs for low powered, high performance products relatively low.
In conventional doping applications, the dopant, phosphorus (P), exhibits high levels of segregation as Germanium (Ge) concentrations are varied. The overall dopant profile resulting from conventional doping methods is not very sharp, but in fact relatively broad. The steepness or sharpness of the resulting curve due to phosphorus segregation is typically about 20 nanometers per decade (20 nm/dec). High P segregation adversely affects important transistor characteristics such as the gain, Early voltage, voltage between the base and collector, and cutoff frequency. Accordingly, SiGe transistors made in accordance with conventional doping methods exhibit relatively poor RF and DC performance. Moreover, other dopants, such as arsenic, exhibit high migration properties, especially in patterned wafers or oxide windows. Thus, such single crystal base structures result in extremely low doping.
There is, therefore, a need in the art for an ALD system in which there is low segregation of a dopant and less sensitivity to temperature and exposure time. There is also a need for an improved system for producing high performance SiGe based HBTs with ALD.